The selection and use of materials and configurations in load bearing structures has become increasingly challenging in recent times due in part to advances in technology and the continuous development of new and better materials. Correspondingly, considerations and expectations involved in such a selection has grown as well, including such traditional considerations as strength and stiffness, but also including demands for more durable, long-lasting, economic and environmentally friendly materials as well. Despite these more recent demands, traditional structural members are still predominant in many current industrial sectors for various reasons, including the fact that the current state of the art has not been able to provide a product that adequately meets these existing demands.
For instance, in the railroad industry, modern railroad ties (also known as cross ties or sleepers) are load bearing structural members used as a base for railroad tracks, providing lateral support and stability for the rails; however, despite decades of technological advancement, railroad ties are still typically made of wood. In fact, in 2006 approximately 18.4 million railroad ties were replaced in the United States alone. Of these, 91.1% were wood, 7.7% were concrete, 0.8% were composite, and 0.4% were steel. This current trend of maintaining railroad ties made of wood is predominantly due to the traditional advantages of the use of wood railroad ties over other types of available railroad ties.
The advantages of continuing to use conventional railroad ties in the United States railroad industry mainly revolve around their consistent and well-known performance characteristics as well as the fact that they are relatively inexpensive to purchase and initially install. Clearly the performance characteristics of modern, wood railroad ties are well-known due to their extensive use throughout the industry dating back to the early years of the railroad industry itself. That is, not only is the life of the wood tie relatively straight-forward to predict, but the strength and stiffness of replacement ties roughly correspond to those of existing wood ties that may be closely situated but not yet needing replacement. This, in turn, allows replacement of only those ties that need to be replaced rather than having to replace an entire section of track ties merely because the performance characteristics of replacement ties is substantially different than those of existing ties. Moreover, the initial purchase and installation of wood ties are relatively inexpensive, not only because of the cost of the wood itself, but also because of the ready availability of the ties and the existing means for their installation.
However, the use of conventional wood railroad ties have a number of disadvantages associated therewith as well. First and foremost, despite their predictability, wood ties must be replaced every few years making such use somewhat expensive over a long period of time. Furthermore, wood's natural susceptibility to certain environmental conditions results in a much greater replacement frequency in some areas compared to others. For example, in a mild, relatively dry climate, a wood railroad tie typically needs replacement every fifteen years; however, in warm, wet areas (particularly those native to wood destroying pests, such as termites) a wood railroad tie might need replacement as frequently as every five years. Thus, one can easily see the long-term economic need for a railroad tie comprised of longer lasting more durable materials.
In an attempt to combat the harsh effects of the environment and, in particular, wood destroying insects, wood ties are typically treated with preservatives. However, the typical preservatives currently used to treat wood ties are either prone to leaching and dissipating from the wood into the surrounding environment and/or are hazardous in nature. For instance, borate products are known for their ability to preserve wood when used as a fungicide and pesticide; however, it is also well known that the borate treatments tend to leach out of treated wood very quickly, particularly along crevices or checks in the surface of the wood itself. In addition, creosote is currently used as a surface treatment to wood railroad ties and utility poles to preserve the wood and prevent deterioration due to insects; however, creosote is a known carcinogen, harmful to humans and animals. In fact, the Environmental Protection Agency considered banning creosote in 1987, but the agency allowed its continued use because of the lack of suitable alternatives. Additionally, a number of lawsuits have been filed in recent years relating to the detrimental effects of creosote usage. Therefore, it is clearly undesirable not only to manufacture such railroad ties, but also to have such railroad ties in use and finally, to dispose of such ties as well.
In an effort to provide a railroad tie with increased longevity, railroad ties of varying material composition have developed over the years; however, none of them have had great success in replacing wood ties because of the various drawbacks associated with their respective use. Steel ties, for instance, have realized only limited use due to a number of factors. First, although steel is a strong, stiff material (with similar properties to wood), steel's susceptibility to rusting makes it an unattractive alternative. Additionally, the use of steel ties is also associated with a high noise level during use. Moreover, since the current method of locating the positions of trains throughout a particular railway system is by utilizing the steel rails for transmitting an electrical signal, the steel rails must be electrically isolated from one another, clearly precluding the use of steel ties between the rails. Finally, the use of steel ties also does not facilitate the typical system of attaching rails to ties through the use of railroad tie spikes. Therefore, steel railroad ties have emerged as an unattractive option to traditional wood ties.
Another option to wood railroad ties that has emerged over the years is railroad ties comprised substantially of concrete. Again, although concrete potentially extends the duration of ties, it also has a number of disadvantages associated therewith. Concrete ties are significantly more expensive than wood ties. This is true, not only in the original purchasing phase, but also in the installation phase. That is, since concrete is significantly heavier than wood, such ties are not able to be handled, transported, or installed using existing processes and equipment. Furthermore, the use of concrete ties requires much more complex rail fastening systems, again increasing the cost of installation. In addition, because of the nature of the repeated compressive and tensile loading a railroad tie experiences, steel reinforcement bars are typically needed. The use of such reinforcement potentially short-circuiting the aforementioned train locating system as well. Finally, the performance characteristics of concrete (particularly hardness and stiffness) are significantly different than those of wood. Because of this difference, when concrete ties are mixed into sections with existing wood ties, uneven loading occurs, resulting in significantly shorter lifespan of the surrounding wood ties. For this reason, entire sections of railroad ties must be replaced when replacing with concrete ties, again resulting in much higher installation costs. Thus, concrete ties are also considered an unattractive alternative to wood railroad ties.
From this background a number of composite railroad ties have emerged in an effort to obtain a longer lasting alternative to a wood tie that maintains the beneficial performance characteristics of a wood tie. Two competing challenges exist in the development and production of composite railroad ties: product performance and economics. That is, virgin resin material could be used to provide the desired performance characteristics; however, the final product would be many times the cost of a wood tie. In contrast, pure recycled polymers could be used to reduce the cost of the tie, but the material properties of such a railroad tie would not be acceptable. Therefore, the typical composite tie composition that has developed to best achieve both parameters is through the use of a material combination of recycled plastic and a mineral additive. However, although the desired properties with such a composition may be obtained, it is difficult to maintain such requirements throughout extensive production runs. High quality, durable recycled plastics are necessary to achieve sufficient strength. Moreover, manufacturing such composite ties consumes tremendous volumes of recycled plastic requiring the maintenance of multiple sources. However, even slight variation in the quality of the recycled plastics not only has an impact on the properties of the polymer, but it also impacts the interaction between the polymers and the mineral additives. All of this leading to railroad ties that may last five times longer than wood railroad ties but that are still unacceptable because of they lack the consistent strength and stiffness properties of wood ties.
Other approaches have been taken for providing composite load bearing members as well including those disclosed in U.S. Pat. Nos. 6,766,963, 6,659,362, 6,336,265, 6,247,651, 6,179,215, 5,886,078, 5,722,589, 5,609,295, 4,286,753, While all disclose a novel approach in solving the problem, each has its own respective disadvantages as well. For instance, U.S. Pat. No. 6,336,265 issued to Neidermair discloses a railroad tie comprised of a slightly undersized wood railroad tie with an end cap attached to each end and coated in a thin plastic film in an effort to retain the performance characteristics of the wood tie, while extending its life through surface protection. However, Neidermair has disadvantages in at least three areas. First, the method of producing the composite tie in Neidermair is a complicated and potentially expensive one, involving the removal of all treated surfaces from an existing wood railroad tie, machining the ends to fit the end caps, and feeding the complete assembly through an extrusion process. Additionally, the plastic coating applied to Neidermair is relatively thin and potentially easily penetrated through typical use and environmental conditions allowing a path for moisture or insects to the untreated wood inside. Finally, the use of a traditional railroad spike is either precluded, or if used, provides an additional path for moisture or insects to the untreated wood tie inside.
Additionally, an attempt at a reinforced composite load bearing structure is disclosed in U.S. Pat. Nos. 5,609,295 and 5,722,589 issued to Richards. The Richards patents disclose a composite member comprising a binding constituent and an aggregate material potentially including an inner strengthening member. However, the Richards patents also have a number of drawbacks associated therewith, particularly when used as a railroad tie. First, the fact that the member is composed mainly of aggregate material leads to both weight and performance characteristics more similar to those associated with existing concrete ties as opposed to existing wood ties. That is, the weight of such a tie precludes traditional transportation and installation methods as used on wood ties, and the increased density of such a tie precludes its use alongside existing wood ties because it would lead an uneven loading condition, as previously described. Finally, existing rail connecting methods could not be used with such a tie either; rather, a more complex system of inserts and/or bolting methods must be used, as described in the Richards patents.
In addition to the foregoing, another significant example of the long felt need for an economical solution to traditional load bearing members is in the marine timber or marine pile industry. Marine timbers are load bearing members that provide columnar support for marine piers and other marine structures. As in the railroad industry, traditional chemically treated wooden marine timbers are still the norm in the industry despite the technological advancements made in composite and other materials. Similarly to traditional railroad ties, wooden marine timbers suffer from a number of drawbacks, in particular the frequent need for replacement due to the wear and tear of harsh environmental conditions. Attempts have been made in the industry to produce longer lasting composite marine timbers through the use of recycled plastics reinforced by fiberglass rebar; however, although these timbers may provide longer life than traditional chemically treated wooden timber, they are significantly more expensive to produce and further dependent on a consistent flow of high quality virgin or recycled polymers. Hence the long-term, economic feasibility of such members is far from sound.
In view of the limitations associated with the prior art, a substantial need exists for a composite load bearing member that has similar properties to that of a traditional wood load bearing member, has a much longer life than that of a traditional wood load bearing member, and is relatively inexpensive to manufacture. Applicant's invention, through a novel combination of component materials and configurations, provides such a load bearing structure.